The NMOS NAND Gate Circuit
Figure 3.22(a) shows a twoinput NMOS NAND gate circuit. This circuit is a modification of the NAND gate using mechanical switches shown in Fig. 3.22(b). The mechanical switches of Fig. 3.22(b) are replaced with NMOS transistors in Fig. 3.22(a). NMOS transistors T_{2} and T_{3} are of the enhancement type and T_{1}, which acts as the load resistance, is of the depletion type. Note that this load is located on top of the switching transistors T_{2} and T_{3 }to produce inversion. If A = 0 and B = 0 (i.e., A and B have 0volt inputs), transistors T_{2 }and T_{3} will remain OFF. Then, the output Z = +V_{DD} (logic 1). The same situation prevails, when A = 0 and B = 1, or B = 0 and A = 1. Now, if A = B = 1, then both T_{2} and T_{3} will conduct and the output Z = 0. These logic operations are summarized as given in Table 3.3, which suggests that the gate shown in Fig. 3.22(a) is a positivelogic NAND gate. To get a negativelogic NAND gate, we replace the NMOS transistors with PMOS transistors.
Figure 3.22(a) shows a twoinput NMOS NAND gate circuit. This circuit is a modification of the NAND gate using mechanical switches shown in Fig. 3.22(b). The mechanical switches of Fig. 3.22(b) are replaced with NMOS transistors in Fig. 3.22(a). NMOS transistors T_{2} and T_{3} are of the enhancement type and T_{1}, which acts as the load resistance, is of the depletion type. Note that this load is located on top of the switching transistors T_{2} and T_{3 }to produce inversion. If A = 0 and B = 0 (i.e., A and B have 0volt inputs), transistors T_{2 }and T_{3} will remain OFF. Then, the output Z = +V_{DD} (logic 1). The same situation prevails, when A = 0 and B = 1, or B = 0 and A = 1. Now, if A = B = 1, then both T_{2} and T_{3} will conduct and the output Z = 0. These logic operations are summarized as given in Table 3.3, which suggests that the gate shown in Fig. 3.22(a) is a positivelogic NAND gate. To get a negativelogic NAND gate, we replace the NMOS transistors with PMOS transistors.
Table 3.3 NMOS NAND
A

B

Z

0
0
1
1

0
1
0
1

1
1
1
0

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